Wave signaling system



Oct. 4, 932.

w. A. M DONALD WAVE smmnme SYSTEM Filed Dec. 1, 1950 2 Sheets-Sheet 1 Mym 1A A m WWW W,

4, 1932. w. A. M DoNALD 4 WAVE SIGNALING SYSTEM Filed Dec. 1, 1930 2Sheets-Sheet 2 500 vo'oo I550 FREQUENCY-lf/LQCYCLEJ FE/FJECO/VO IINVENTOR ATTORNEY Patented Oct. 4, 1932 UNITED STATES PATENT OFFICEWILLIAM A. MACDONALD, 01' LITTLE NECK, NEW YORK, ASSIGNOR TO HAZELTINECORPORATION, OF JERSEY CITY, NEW JERSEY, CORPORATION OF DELAWARE WAVESIGNALING SYSTEM Application filed December 1, 1930. Serial No. 499,229.

This invention relates to high frequency signaling, and moreparticularly to systems especially adapted to the selection andamphfication of radio-frequency waves.

The principal object of the invention is to obtain in a radio-frequencyamplifing system, a high degree of sensitivity, selectivity and fidelityand at the same time to reduce the tendency toward oscillation of therepeating elements of the amplifier.

Broadly stated, the system by which the above object is accomplishedcomprises an untuned radio-frequency amplifier preceded by a tuning, orband-frequency selective, system comprising a plurality of tunedcircuits coupled by the proper amount of coupling to providesubstantially the maximum possible transfer of energy from one tunedcircuit to the next, consistent with a band transmission characteristicsuited to freely transmit all the important frequencies of the sidebandsand to discriminate against all other frequencies. Such a band selectivesystem is disclosed in my copending application Se rial No. 469,487,filed July 21, 1930.

A feature of the invention is the radio-frequency amplifier, which is ofthe untuned, that is, untunable, type and has interstage coupling meansso designed that each stage is non-regenerative. The amplifier repeatingelements are preferably coupled by impedance coupling arrangements whicheffectively place capacitive loads upon the output circuits of eachrepeater over the tuning frequency range. These capacitive reactancesare preferably provided by a choke coil or transformer connected betweenthe anode and cathode of each repeater. The chokes or transformers areso constructed that their distributed capacity resonates with theirinductance at a frequency slightly below the lowest frequency of thetuning range; there.- i by causing the chokes or transformers to beeffectively capacitive throughout the tuning range. It is understood, ofcourse, that the distributed capacity includes that of the associatedamplifying tubes and any associated circuit elements. I

There are certain advantages inherent in an untuned radio frequencyamplifier, partieularly when the output impedance connected to arepeating element is capacitive. One advantage resides in the fact thatthe coupling impedances of the amplifier can be made resonant somewhatbelow the tuning range, thereby causing the voltage transfer to thesucceeding stage to be a maximum at the low frequency range. At the highfrequency range the voltage decreases with increased frequency. Thatthis is an advantage follows from the fact that most tuning or selectivesystems which must, of necessity, be associated with a radio-frequencyamplifier, are characterized by an energy transfer which increases withincreasing frequency. By properly proportioning the coupling elements ofthe amplifier, then, compensation of the characteristic of the tuningsystem may be efi'ected, and the overall response characteristic of theamplifier and selective system may be made uniform or at least regulatedwithin wide limi A second important advantage in the use of an untunedradio-frequency amplifier is the ease with which oscillation can beprevented. It is well known that when the impedance connected to theoutput of a vacuum tube amplifier is capacitive, any voltage fed backfrom the output to the input of the amplifier is in the wrong phase toaid the input voltage, and hence the condition required for oscillationis non-existent. In consequence, when the output impedance is made toresonate below the tuning range, it is capacitive throughout that rangeand not conducive to oscillation.

Another feature .of the invention is the manner of coupling the severaltuned circuits of the tuning, or preselecting, system so that 1tsresponse characteristic compensates for that of the amplifier, or givesto the receiver a desired over-all characteristic. It is contemplated toregulate the characteristic of the tuner by the proper combination ofmag netic and capacitive coupling between the tuned circuits.

The combination of an untuned amplifier with a preselecting systempossesses advantages not found in an ordinary tuned radiofrequencyamplifier. In the first place, the

presence of the several tuned circuits, ahead of the first repeatingelement efiectively suppresses currents of undesired frequencies andsubstantially prevents harmful modulations which produce cross-talk.Furthermore there is attained a high de ree of selectivity together withhigh amplification and freedom from oscillation.

A further and more com lete understanding1 of the invention may e hadfrom the fo owing detailed description and the accompanymg drawings, ofwhich:

Fig. 1 illustrates an untuned radio-frequency amplifier connected to theoutput'of a three-circult tuning, or pre-selecting, stem of Iiwhich thecircuits are coupled inductively;

1g. amplifier and a preselecting system, which is similar to that ofFig. 1 except that the.

circuits of the pre-selecting system are coupled .capa'citively insteadof inductively;

Fig. 3 illustrates an arrangement similar tothose of Figs. 1 and 2, inwhich the circuits of the preselector are coupled both inductively andcapacitively;

Fig. 4 illustrates the'combination with a preselecting system of anuntuned amplifier comprising screen-grid tubes;

Fig. 5 shows the variation over the tuning and C5,. The tuned circuit 10is coupled to, tuned circuit 11 by a small inductance L in series with LL being magnetically associated with coil L of circuit 11. Likewisecircuit 11 is coupled to circuit 12 by a small inductance L in serieswith inductance L The small coupling inductances L and L preferably areconstituted by one or two turns of wire and are located at the lowpotential ends of inductances L and L ,'respectively, so thatsubstantially optimum coupling is obtained. Optimum coupling as usedherein, means the degree of coupling which results in the maximumtransfer of energy from one tuned circuit to the next while maintainingbut a single resonant peak; the order of this optimum coupling isaboutone or two per cent.

To minimize undesired electro-static cou-" pling between the severaltuned circuits of the pre-selector, metal shields 13, 14 and 15, shownby dotted lines, are arranged to eifecshows a combination of a untunedinductances, it should be understood that the coupling may be obtainedwithout these additional coils slmply by so situating inductances L Land L that the necessary co-efiicient of coupling between them isobtained.

' )The three circuits 10, 11 and 12 are tuned by means of the variablecondensers C C2,

and C respectively and the circuits are simultaneously tuned to the samefrequency by gauging the condensers in a uni-control arng between thesuccessive circuits-is have been shown as rangement as indicated by thedotted line 40. The radio-frequency amplifier, which is connected to theoutput of the pre-selector, is

of the untunable type and comprises three thermionic repeaters, 16, 17and 18, joined in tandem.- In practice, the output terminals 1 and 2 ofthe amplifier are connected to a detector and voice frequency amplifyingsystem, but for convenience the detector and voice frequency amplifierare omitted as are also the power supply gources for thethermionicrepeaters, inasmuch as these are wellknown and constitute no part of theinven-- tion.

The untunedcoupling arrangements be tween the amplifier tubes are of theimpedance type. Considering the coupling between the repeating tubes 16and 17, the arrangement comprises a radio-frequency choke coil 19connected between the anode'of tube 16 and the positive terminal ofthe'B-battery. A blocking condenser 21 is connected between the anode oftube 16 and the control electrode of tube 17. The usual high gridleakresistance 22 is connected between the control electrode and the cathodeof tube 17. The untuned coupling arrangement between tubes 17 and 18 isvery similar to that betweentubes 16 and 17 except that. the anode lead'of tube 17 is tapped into the coupling coil 20 b means of a variablecontact, thus effective y making coil 20 an auto-transformer.

' The couplin coils 19 and 20 are preferably wound with fi ne wire andsupported upon cylindrical forms although this construction is notessential. lSy winding the coils in this manner, however, thedistributed capacity of the coils is reduced to an extremely low value.The coils, however, may be constructed by any of the well knownmethodsfor securin low distributed capacity. The coupling coi s 19 and20 are characterized by being capacitively reactive over the entiretuning rangeof frequency that is, the inductance is sufiicient- 1y largeso that it resonates with the distributed capacity of the winding, theoutput capacity of the associated tube and the input capacity of thenext succeeding tube at a frequency lower, but not greatly lower, thanthe lowest frequency of the tuning range. As a pling arrangement ishigher for the lower frepling coil L quenci'es of the tuning range thanfor the higher frequencies; hence the ratio of output to input voltageof a tube and its associated coupling arrangement is higher for the lowfrequency range than for the high frequency range. At the same time thecapacitive load of the plate circuit shifts the phase of the outputcurrents sufliciently so that the internal plate to grid coupling of thetube does not cause positive re generation.

In the chart of Fig. 5, the ordinates indicate voltage ratios and theabscissas represent frequencies of the ordinary broadcasting range, thatis,- from 500 to 1500 kilocycles per second. Curve a illustrates atypical volt age ratio or response characteristic of one of the untunedamplifier coupling systems. The curve shows that the highest voltage isdeveloped at the lower frequencies, that is, near the resonant frequencyof the coupling coil. At the high frequency end'the voltage ratio isonly about one-half of that at the low frequency end.

The signals are impressed upon the first tuned circuit ofthe preselectorby an antenna A, the lead from which is preferably tapped into coil L afew turns from the low potential end. Although in the schematic circuitdiagram, coil L is shown as an autotransformer, a two-windingtransformer may be employed with equal facility.

The first tuned circuit composed of inductance coil L and variablecondenser C is tuned to resonance and the-resonant current istransferred to the second tuned circuit composed of inductance coil Land variable capacity C 'by means of the small cou- The second tunedcircuit is likewise tuned to resonance and the energy conveyed therefromto the third tuned circuit composed of inductance coil L and variablecapacity C by means of the coupling coil L The third tuned circuit issimilarly tuned to resonance and the desired signal voltage applied tothe input of the amplifier. When the tuning condensers are gangedtogether, as illustrated, the three circuits are simultaneously tuned bya uni-control device. .The high potential terminal 3 of the preselectoris adjustable so that any desired signal voltage may be impres'sed uponthe untuned amplifier. This variable connection, although desirableunder some conditions, is not necessarily essential to the invention.Tapping in across a portion of the third tuned circuit in the mannerdescribed has been found to be particularly efiective when the negativeregeneration of the untuned amplifier is high, for withthis-connection asmaller loading effect occurs in the third tuned circuit; hence, theselectivity is better than it might otherwise be. I

In addition to the loading effect resulting from the negativeregeneration of-an untuned amplifier, the adjustable connection of thesystem from the preselecting system to the amplifier furnishes a verysatisfactory method of sec-urin volume control, for as the highpotential Tead is moved toward the grounded end of inductance L thevoltage across the output terminals of the preselector diminishes, thuseffecting control of the signal voltage.

It is not essential that the signal voltage to the amplifier be suppliedby an auto-trans-' former connection such as terminal 3 of coil L but itmay be suppliedto the amplifier with equal facility by means of atwo-winding transformer of which the mutual inductance between theprimary and secondary is made variable to accomplish the desired volumecontrol.

A tuning system composed of coupled tuned circuits alone, such as thatof Fig. 1, in which the tuned circuits are coupled magnetically, ischaracterized by a response whichrises with increasing frequency overthe tuning range. When the antenna is naturally resonant above thetuning range, as is usual y the case, its impedance is effectivelycapacitive; which causes the voltage delivered by the antenna to theselector system to increase with increasing frequency. Since the voltagecharacteristics of the antenna and of the magnetically coupled tunerboth slope upward with frequency, the over-all responsecharacteristic ofthe antenna and preselecting system of Fig. 1 rises with frequency asshown by curve b of Fig. 5.

Fig. 2 illustrates a radio-frequency recelving system which is ingeneral slmilar to that of Fig. 1. The preselector, however, instead ofutilizing magnetic coupling between successive tuned circuits, employscapacltlve coupling. The preselector comprises three tuned circuits 25,26 and 27, each including an inductance tuned by a variable capacityasin the preselector of Fig. 1. The tuned circuits are coupled only bycoupling con-- densers C and C 1 In the drawings each coupling condenseris shown as being connected between a tap in one tuning coil and thehigh potential end of the next succeeding tuned circuit. It 1s notessential that the coupling condensers be tapped into the coils in thismanner, but it is often convenient to follow this expedient for withthis connection the coupling capacities 1L may be larger and are notrequired to have such precise values, as would be the case if the highpotential ends of all succeeding tuned circuits were directly coupled bythe capacities.

The pre-selector systemof the receiver shown in Fig. 2 is characterizedby a voltage ratio or response which increases toward the higher end ofthe tuning range. This increasing response results from the fact thatcoupled pre-selector system, includin 1 ly and capacitively. selector,it comprises three tuned circuits, 30', I 31 and 32. Circuit 30 ismagnetically coupled the coupling capacities convey more current athigher than at lower frequencies. A typical response characteristic ofthe capacitivellly t e antenna, is illustrated by curve of i 5.

Due to the effect of the capacitive couphng,

to circuit 31 by a small coil L and circuit 31is magnetically coupled tocircuit 32 by a similar coil L Coils L 'and L correspond-to' coils L andL respectively of Fig. 1 and may be constructed and arran d in the samemanner. In addition to being magnetically coupled the tuned circuits arealso coupled. capacitively by means of condensers C and C14 n the mannerof the capacitive coupling of the re-selector ofFig. 2. Condensers C andmay be tapped respectively into coils L and L in the man- Q nerexplained in connection with the pre select r of Fig. 2, if desired.

'Th dualistic coupling between the seve'rhl tuned circuits of thepre-selector of Fig. 3

may be made use of to provide the receiving 35 system with desiredresponse characteristics.

The capacitive and magnetic couplings may be arranged to mutually aid orto mutually oppose each other dependhig upon the result desired. Theaddition or the subtraction. of the coupling effects makes possible analteration of the volta ratio, or. response characteristics of t epre-selective system within wide limits. For example, the magneticallycou'pled pro-selectors have a substantially uniform response over thetuning range, while the effect of the capacitive coupling is to transferthe reatwt energy at the higher frequencies. If 1t is desired,therefore, to obtain an extremely stee gain characteristic between theinput an the output of the tunin system, favoring the higherfrequencies, t e capacity coupling shouldfibe arranged to aid themagnetic cou ling. If, on the other hand, a more uni orm gaincharacteristic between the input and the output of the tuning system isdesired, the capacity coupling should be arranged-to oppose the imaignetic coupling.

ig. 4 illustrates a radio-fre uency receiving system similar to that ofthat the untuned amplifier utilizes four-element amplifying tubes of thescreen-grid type. instead of. three-element tubes shown in Fig. 3. Theuseof screen grid tubes is often advantageous because of the highersystem ig. 3 except amplifications that can be secured as com paged withthe conventional three-element tu es.

Fig. 6 shows the over-all gain characteristic of a receiver employingan'antenna, a

selector system composed of three tuned circuits and an untunedam'plifiercomposed of a several stages of repeating elements havingcapacitively reactive loads in the output circuits. The res onsecharacteristic shown in this figure is o tained by designing the antennaand preselector system to have a voltage ratiocharacteristic which iscomplementary to that'of the'unttmed amplifier; hence,

the overall characteristic is substantially uniform overthe tuningrange. The importance of being able to alter the slope of the respgnsecharacteristic of the selectin comes apparent when it is consi ered thatthe slope of the characteristic of the untuned am lifier may not liewhollywithin the'contro of the designer. For exam 1e, it. may benecessary to employ ampli ing tubes having either extremely small orextremely large input and/or output capacities; and the amphficationcharacteristic will depend upon the tubes employed. The slo ofthe resonse characteristic of a m ti-stage ampl' er will vary in accordanceover-all response, or voltage ratio, of the amplifier will be theproduct of that bf the several stages. For. example, if thecharacteristic of a single stage varies from an amplification factorthree at the high frequency end to six at the low frequency end, then iffour stagesof amplification be emplo ed, the slope of the over-allamplification c aracteristic at the frequency limits will be in theratio of' 16 to 1. In order to provide a substantially uniform gaincharacteristic for the completed. radio-frequencyreceiver, it isnecessary to slope the again characteristic of the antenna andpreselector system in the opposite manner to that of the amplifier andto provide a slope having an amplification .at the ends of thetuningrange in the ratio of 1 to 16.

What is claimed is:

1. In a radio receiving system, in com-- bination, an untunableradio-frequency amplifier and a frequency-selecting arrangementcomprising a plurality of coupled tunable circuits, said amplifiercomprising a thermionic repeater having an output load which iscapacitive over/the receiving frequency range whereby said'amplifieroperates in a stable condition, said selecting arrangement beingconnected in tandem, ahead of said amplifier whereby said system isselective and free from cross-talk. 2. A radio receiving system inaccordance with claim 1 in which said tunable circuits are coupledelectro-magnetically.

3. A system in accordance with claim 1 in which said tunable circuitsare coupled 1 both electrmmagnetically and electro-statically. I i

- 4. A system in accordance with claim 1 in which said tunable circuitsare coupled both electro-magnetically and electro-statically in phaseopposition, whereby the voltage ratio between the input and output ofsaid plurality of tunable circuits is approximately uniform over thetuning range.

5. A system in accordance with claim 1 in fier, and means for variablyadjustin the impedance of said inductive coupling, w ereby thestrengthof signal voltages supplied to said amphfier may be re lated.

In testimony whereof I a my signature.

WILLIAM A. MACDONALD.

which said tunable circuits are coupled elec- I tro-magnetically andelectro-stati'cally, said electro-static and electro-magnetic couplingbeing arranged to mutually aid each other,

whereby the ratio between the input and out- I put voltages of saidplurality of tunable circuits varies oversaid frequency range at a ratemore rapid than would if said plurality of tunable circuits were onlyelectromagnetically coupled.

' comprising a plurality of tunable circuits 6. In aradio receiver apreselector comprising a plurality of variably tuned cir-. cuitsarranged in tandem and coupled to- 7 In a radio receiver, including aseries.

of resonant circuits tunable over a certain frequency range, canuntunable amplifier comprising at least one stage of amplification, acoil connected to the output circuit of said amplifier stage having suchvalueof inductance that it is resonant at a frequency fixed lower, butnot greatly lower, than the lowest frequency of said range, whereby theratio of output to input voltage of said amplifying stage decreases withincreased frequency, and grounded metallic shields interposed betweenthe series of tunable resonant circuits whereby undesired couplingbetween said series of tunable circuits is reduced.

8. In a radio receiver a tuning system arranged to be syntonously tunedover a range in frequency, an untunable amplifierstage comprlsing avacuum tube having an anode circuit and a load in the anode circuit,

which is capacitively reactive throughout the tuning range offrequencies, a mutually inductive coupling between the output of saidtuning system and the input of said ampli-

